1. FIELD OF THE INVENTION
This invention relates to an amplifier circuit and, more particularly, to a protection operation in a case where an output line from the amplifier circuit is short-circuited to a power supply (Vcc) or ground side of a stabilized power supply circuit for supplying power to the amplifier circuit.
2. DESCRIPTION OF THE RELATED ART
For example, this kind of amplifier circuit is used with a sensor mounted in a motor vehicle or the like. FIG. 6 is a block diagram showing a part of an electronic controller of a motor vehicle. A sensor unit 110 for detecting pressure, temperature and other quantities through a power supply line Vcc, a ground line GND and a signal line S is connected to a control unit 100 (system side) which includes a microprocessor (MPU) 101 and other components. A signal from the sensor unit 110 representing a result of detection is amplified by an amplifier circuit 5 before it is supplied to the control unit 100. Amplifier circuits in accordance with the present invention are used, for example, like this amplifier circuit 5. In more detail, a voltage of, for example, 5 V is supplied to the sensor unit 110 and the MPU 101 from a stabilized power source 3 of the control unit 100 through the power supply line Vcc. The detection signal from the sensor unit 110 is supplied from an output terminal 7 of the amplifier circuit 5 to the MPU 101 of the control unit 100 through the signal line S. In this kind of unit, there is a possibility of the signal line S being short-circuited to the power supply line Vcc or the ground GND by being connected thereto in a connector 111 of the sensor unit 110, as indicated at x in FIG. 6. The MPU 101 determines that the sensor unit 110 is malfunctioning, when the output from the sensor unit 10 is 0.5 V or lower, or 4.5 V or higher (in a failure range). However, it is possible that the output from the sensor unit 110 is out of the failure range even when a short circuit in the connector 111, such as that described above, occurs. In such a situation, the short circuit cannot be detected on the control unit 100 side. A resistance 8 in the control unit 100 represents a load.
FIG. 7 is a schematic block diagram of a circuit system including this kind of amplifier circuit. A minus terminal of a power source 1 is connected to ground 2, and a stabilized power supply circuit 3 is connected to a plus terminal of the power source 1. The stabilized power supply circuit 3 has a stabilized power supply line 4 (V.sub.CC), which is connected to a semiconductor amplifier circuit 5. An input signal power source 6 generates an input signal to the amplifier circuit 5. A load 8 is connected between an output terminal 7 of the amplifier circuit 5 and V.sub.CC 4. An operational amplifier 9 constitutes the amplifier circuit 5. An input resistor 10 is connected between an inverting input terminal of the operational amplifier 9 and the input signal power source 6. A feedback resistor 11 is connected between the inverting input terminal and an output terminal of the operational amplifier 9. A reference power source 12 is connected to a noninverting input terminal of the operational amplifier 9.
FIG. 8 is an equivalent circuit diagram of the operational amplifier 9 of the conventional amplifier circuit. The base of a PNP transistor 13 corresponds to the inverting input of the operational amplifier 9, and the base of a PNP transistor 14 corresponds to the noninverting input of the operational amplifier 9. The emitters of the PNP transistors 13 and 14 are connected to each other to form a differential input pair. A current source 15 is connected between the emitters of the PNP transistors 13 and 14. An NPN transistor 16 has its collector and base connected to the collector of the PNP transistor 13, and an NPN transistor 17 has its collector connected to the collector of the PNP transistor 14. The NPN transistors 16 and 17 have their bases connected to each other and their emitters connected in common to ground 2 to form a current mirror circuit.
An NPN transistor 18 has its base connected to a point of connection between the collectors of the PNP transistor 14 and the NPN transistor 17, and its collector connected to V.sub.CC 4 for emitter-follower operation. An NPN transistor 19 has its base connected to the emitter of the NPN transistor 18 and its emitter connected to ground 2 to form a common-emitter circuit. A constant-current source 20 is connected between the collector of the NPN transistor 19 and V.sub.CC 4. A resistor 21 is connected between the collector of the NPN transistor 19 and the output terminal 7. A capacitor 22 is connected between the base of the NPN transistor 18 and the collector of the NPN transistor 19.
The operation of the system-will be described below. If the voltage of the input signal from the input signal power source 6 is V.sub.I, the resistance value of the input resistor 10 is R.sub.I, the resistance value of the feedback resistor 11 is R.sub.F, then an output voltage V.sub.O of the operational amplifier 9 is expressed by the following equation: ##EQU1##
Since the output terminal of the amplifier circuit 5 is connected to the output terminal of the operational amplifier 9, the output from the amplifier circuit 5 is V.sub.O expressed by the equation (1).
If the impedance of the load 8 is Z.sub.L, a current I.sub.O which flows into the operational amplifier 9 through the output terminal 7 is ##EQU2## and, if the current in the constant-current source 20 is I.sub.R, the collector current I.sub.C19 in the NPN transistor 19 is EQU I.sub.C19 =I.sub.O +I.sub.R ( 3)
If the resistance value of the resistor 21 is R.sub.O, a saturated collector-emitter voltage of the NPN transistor is V.sub.S, a minimum value V.sub.OL of the output voltage is EQU V.sub.OL =I.sub.O R.sub.O +V.sub.S ( 4)
When the minimum value V.sub.OL is expressed by equation (4), the current flowing in through the output terminal 7 is maximized. The maximum value I.sub.O (max) of the current I is expressed by the following equation obtained by substituting equation (4) in equation (2): ##EQU3##
As can be understood from equation (5), it is desirable to reduce R.sub.O when a larger value of I.sub.O (max) is required. Also, as can be understood from equation (4), V.sub.OL becomes smaller an the output range (dynamic range) of the operational amplifier 9 extends, if R.sub.O is reduced.
For example, when V.sub.CC =5 V, V.sub.S =0.3 V and I.sub.O (max)=2 mA are set, R.sub.O =100.OMEGA. and Z.sub.L =2.25 k.OMEGA. if V.sub.OL =0.5 V.
In the above-described circuit, if a portion of the output terminal 7 is short-circuited to ground 2 by, for example, a connector failure as indicated by a broken line a in FIG. 7, a current I.sub.SG caused to flow to ground 2 by the short circuit is expressed by the following equation: ##EQU4##
Accordingly, if I.sub.R =0.5 mA, I.sub.SG =2.7 mA. In this case, there is no possibility of a particular circuit failure, such as a breakdown in stabilized power supply circuit 3 or amplifier circuit 5.
Then, if a portion of the output terminal 7 is short-circuited to V.sub.CC 4 as indicated by a broken line b in FIG. 7, a current I.sub.SV caused by the short circuit is a smaller one of two values determined by the following equations: ##EQU5## or EQU I.sub.SV =I.sub.15 .times..beta..sub.1 .times..beta..sub.2 ( 8)
where I.sub.15 is the current in the constant-current source 15, .beta..sub.1 is a direct current amplification factor of the NPN transistor 8 and .beta..sub.2 is a direct current amplification factor of the NPN transistor 19.
If V.sub.CC =5 V, R.sub.O =100.OMEGA., I.sub.15 =5 .mu.A, and .beta..sub.1 =.beta..sub.2 =100, then I.sub.SV =50 mA from equation (7), and I.sub.SV =50 mA from equation (8). Consequently, I.sub.SV =50 mA (since the values of equations (7) and (8) are equal to each other in this case).
Thus, it is possible that a short circuit between the output terminal 7 and the power supply line (V.sub.CC 4), such as that as indicated by the line b in FIG. 7, causes a current which is ten times or more greater than a current caused by a short circuit to ground 2 indicated at a, and which may cause a breakdown in the amplifier circuit 5.
Ordinarily, an output current limiting circuit is incorporated in the stabilized power supply circuit 3 to protect the same. The limited output current value is changed according to the scale of the circuit connected to the stabilized power supply circuit 3. For example, in the case of a short-circuit as indicated by the line b in FIG. 7, V.sub.CC =3 V, if the current I.sub.SV, which is determined by the smaller one of the values obtained by equations (7) and (8), i.e., the value of equation (7) in this case, is limited to I.sub.SV =Vcc/Ro=30 mA.
That is, in the case of the short circuit indicated by the line b in FIG. 7, the voltage V.sub.CC is reduced from 5 V to 3 V. In this case, the output voltage of the amplifier circuit 5 is equal to V.sub.CC, i.e., 3 V. The voltage of 3 V is also output when the amplifier circuit 5 is operating normally. From this output voltage, it appears that the output from the amplifier circuit 5 is normal even though the state is even though abnormal, that is, there is a short circuit between V.sub.CC 4 and the output terminal 7 of the amplifier circuit 5. In such a situation, there is a possibility of occurrence of a serious failure in a system (not shown) using the output terminal 7 (runaway may occur if a certain feed back is used). Ordinarily, with a microcomputer control or the like, the voltage at the output terminal of the amplifier circuit 5 is not increased to the level of V.sub.CC 4. Therefore, the output terminal 7 of the amplifier circuit may be regarded as an error voltage not to be referred to if, for example, it is 4.5 V or higher. In the situation discussed here, however, the output voltage at the output terminal 7 is 3 V and such an error detection cannot be expected.
As described above, if the output from the conventional amplifier circuit is short-circuited to a power supply voltage V.sub.CC, which is an output from a stabilized power supply circuit, an excessively large current flows and damages the amplifier circuit, or the output voltage V.sub.CC of the stabilized power supply circuit is reduced to cause a serious failure in a system which is connected to the output terminal of the amplifier circuit. Thus, the conventional amplifier is unsatisfactory in terms of reliability.